Patent literature 1: JP 2013-219905A (corresponding to US2013/0264974A1).
Conventionally, a PWM control in a control apparatus of a rotary machine controls a switching timing of a switching element in an electric power converter such as an inverter, so that electricity supply to the rotary machine is controlled. The PWM control generates a duty signal based on a voltage instruction value. The duty signal is a ratio of on/off cycles regarding a switching period. The PWM control turns on and off of the switching element by comparing the duty signal with a carrier wave such as a triangular wave, a saw tooth wave.
In the PWM control, as the PWM frequency, which is a frequency of the carrier wave, is set higher, that is, as a cycle length is set shorter, controllability is improved. For example, patent literature 1 discloses a frequency of a carrier wave may be set to 20 kHz and a cycle length may be set to 50 μs, for example.
The inventors of the present application have found the following.
When the PWM frequency increases, the number of times of switching per unit time may increase and a switching loss may increase. In a normal drive status, in order to improve controllability, a frequency of, for example, 20 kHz is used.
In a control apparatus of a motor that generates a steering assist torque used in electric power steering apparatus of a vehicle, especially when the control apparatus is attached to a steering column, since the control apparatus is disposed close to a driver, so that the driver and a passenger may be less likely to hear an operation sound, it may be preferable to use the PWM frequency of 20 kHz or more for the purpose of silence.
It is supposed that the control apparatus of the rotary machine includes multiple systems of electric power converters, and controls electricity supply to multiple winding groups corresponding to the multiple systems. In addition, it is supposed that an electric power converter or a winding group in any one of the multiple systems gets out of order, and the rotary machine is driven only by a normally operating system. Incidentally, the normally operating system means a system that operates normally. For an electric power steering apparatus, the above situation may correspond to a case where a failure has occurred in either system during travelling, a driver performs an evacuation travel to a vehicle dealer or the like while ensuring an assist function of a steering torque only by the normally operating system. In this situation, it may be desired to improve power efficiency by reducing the switching loss in the electric power converter of the normally operating system and to prevent heat generation, rather than controllability and silence.